The present invention relates to a method for preserving alignment marks, and more particularly for a method used to monitor the use of clamp ring shielding tabs to preserve alignment marks by shielding the alignment marks during deposition of material on a wafer.
The preservation of alignment marks during the deposition of layers of material on a wafer is of primary importance in photolithographic processing of integrated circuit wafers of the background art. One method used to preserve the alignment marks is to place shielding tabs on the clamp ring used to hold the wafer in place during material deposition. The effectiveness of the shielding tabs depends on the location of the shielding tabs when material is deposited, and continual monitoring of this location is important.
U.S. Pat. No. 5,456,756 to Ramaswami et al. and U.S. Pat. No. 5,614,446 to Ramaswami et al., the entirety of each which are hereby incorporated by reference, describe the use of shielding tabs on the clamp ring used to hold wafers in place during material deposition to shield the alignment marks during material deposition. This shielding prevents material from being deposited over the alignment marks and obscuring the alignment marks.
U.S. Pat. No. 5,320,728 to Tepman, the entirety of which is hereby incorporated by reference, describes a planar magnetron sputtering source used to produce a uniform coating of material during material deposition on a wafer. U.S. Pat. No. 6,277,658 to Jeng et al., the entirety of which is hereby incorporated by reference, describes a method of using a monitor wafer with alignment marks to monitor the shielding of alignment marks during material deposition steps.
FIG. 1A shows a top view of an oxide wafer 10, used as a test wafer, having monitor marks 12 formed thereon. The test wafer 10 has the same size and shape as a product wafer and also has the alignment notch 14. The monitor marks 12 are larger than the alignment marks 14 on the product wafer, but the center of the monitor marks 12 are at the same location on the test wafer 10 as the center of the alignment marks on the product wafer. In this example, the alignment marks 12 have the shape of a star. FIG. 1B shows a top view of the clamping ring 20 assembled to the test wafer 10, showing that each shielding tab 22 covers most, but not all, of one of the monitor marks 12.
FIG. 2A shows the star-like monitor marks 12 on the test wafer 10 described with respect to the method of the background art. The monitor marks 12 include a plurality of squares 16 with the same intervals and a mark center 17, which has the same location as the center of the alignment mark on the product wafers. FIG. 2B shows top views of a part of a test wafer 10 after a layer of opaque material 19 has been deposited on the test wafer 10 and the clamp ring has been removed. In FIG. 2B, part of the monitor marks 12 are covered by the deposited material 19. The center square 17 of the monitor mark is well centered in the space protected by the shielding tabs of the clamp ring as can be seen by observing the monitor squares 16 around the center square 17. The monitor squares 16 surrounding the center square 17 can be used to determine the minimum distance 192 to the edge 191 of the deposited material 19. If the minimum distance 192 is greater than a critical distance, the location of the shielding tabs in relation to the oxide test wafer is acceptable and the processing of product wafers can continue.
If the minimum distance 192 is less than or equal to the critical distance, as shown in FIG. 2C, the location of the shielding tabs in relation to the oxide test wafer is not acceptable and corrective action must be taken before the processing of product wafers is continued. However, the aforementioned method of the background art requires specific masks and specific processes to form the monitor marks on the test wafers, thus increasing the cost of the test wafers. Furthermore, the minimum distance can only be approximately determined by counting the monitor marks shown on a monitor, such that the counting results are usually unreliable and easily causes fault operations. Hence, the present inventor has determined that there is a need for a better method that can overcome the aforementioned problems of the background art.